Lecture 35: Review
Final Review
1. Given the reaction NO(g)
1/2O2(g) + 1/2 N2(g)
if during a given time interval the rate of formation of O2(g) is 5.40 x 10-2 mol L-1 s-1
then during this time period )[NO(g)]/ )t is:
a +5.4 x 10-2 mol L-1 s-1
b -1.08 x
10-1
mol
L-1 s-1
c - 5.4 x 10-2 mol L-1 s-1
d +1.08 x 10-1 mol L-1 s-1
}
2 moles of NO go when 1 mole
of O2 forms: reacts twice as fast
e -2.7 x 10-2 mol L-1 s-1
or use aA + bB 6 cC +dD
rate
= -(1/a) x d[A]/dt = -(1/b) x d[B]/dt
= (1/c) x d[C]/t = (1/d) x d[D]/dt
- better to think it through
b,c
35-1
2. Use the following experimental data for NO(g) + Cl2(g)
[NO]
1.01 x 10-2
1.02 x 10-2
2.02 x 10-2
3.02 x 10-2
[Cl2]
5.30 x 10-2
1.05 x 10-1
1.51 x 10-1
1.53 x 10-2
Initial Rate
5 x 10-2
1.99 x 10-1
9.02 x 10-1
1.34
NOCl2 (g)
Final Review
conc x 2
rate x4
the order of Cl2(g) in the rate law is:
a1
b none of these
c2
d3
e0
35-2
3. Tritium is radioactive and decays by a first order process with a half-life of
12.5yr. If an experiment starts with 1.00 x 10-6 moles of tritium how much is
left after 4.5 yrs?
Final Review
a 1.1 x 10-7 mol
b 7.8 x 10-7 mol
c 3.6 x 10-7 mol
d 4.8 x 10-7 mol
e 1.0 x 10-7 mol
k = 0.693/t 1/2 = 0.0554
ln A0/A = k x 4.5 = 0.249
A = 7.8 x 10-7
4. The rate constant for a particular first order reaction is 1.2 x 10-4 s-1 at 500 oC.
The rate constant at 600oC is 6.8 x 10-3 s-1. The activation energy is:
a 27.4 kJ mol-1
b 226 kJ mol-1
c 318 kJ mol-1
d 3.36 kJ mol-1
e 33.6 kJ mol-1
ln (k2/k1) = Ea/R()T/T1T2)
8.314 x 4.03 = Ea (100/773 x 873)
35-3
5.A catalyst :
a increases the order of a reaction
b increases the energy of activation of a reaction
c increases the temperature of a reaction
d increases the rate of a reaction without being consumed
e none of the above
6. The reaction X + 2Y
X + Y
I + Y
Final Review
A + Z occurs via the mechanism
Z + I slow
A
fast
which of the following rate laws is consistent with the mechanism?
a k1 [I][Y]
b k1 [I][Y]2
c k1 [X][Y]
d k2 [X] [Y]2 [Z]-1
e k1k2[X][Y]2/[Z]
35-4
7. Consider: H2O2(aq) + 3I-(aq) + 2H+ (aq)
Final Review
H2O(l) + I3-(aq)
doubling the concentration of I-(aq) without changing the other concentrations would:
a increase the rate of the reaction by a factor of 3
b increase the rate of the reaction by a factor of 8
c increase the rate of the reaction by a factor of 4/3
d increase the rate of the reaction by a factor of 9
e the effect cannot be determined with the information given
8. In collision theory of reaction rates which of the following accounts for the
largest portion of the observed temperature dependence.
a As temperature increases the rate of collisions between reacting molecules increases
b Intermolecular forces increase with temperature
c There are many collisions where the molecules are not oriented correctly to react.
d The electrons in the reacting molecules move faster at high temperatures
e For a collision to result in a reaction there is some minimum
energy that the colliding molecules must possess
35-5
Final Review
long answer
D [10 points]
1 A first order reaction has an activation energy of 49.8 kJ mol-1 When a
catalyst is added the activation energy for the reaction decreases to one
half of the above value.
Calculate the ratio kuncatalyzed / k catalyzed
ku/kc = Aexp (-49,800/8.314 x 298)
A exp (-24,900/8.314 x 298)
= 4.3 x 10-5
kcat is about 23,000 X larger
35-6
long answer (cont...)
Final Review
2. The reaction
A
is second order in A.
B + C
When [A]0 = 0.1053 the reaction is 20.1% complete in 40.1 minutes.
(a) Calculate the rate constant k.
1/[A]- 1/[A]0 = kt
1/0.084 - 1/0.1053 = 40.1 k
k= 0.06 mol -1 L min-1
b. Why is the half-life not very useful in describing a second-order reaction?
t 1/2 = 1/k[A]0
therefore you need the initial concentration
to get at the rate constant. Invariably the initial conc
is unknown...
35-7
page 6 of 6
Final Review
G. [10 points]
1. One proposed mechanism for the destruction of ozone in the upper
atmosphere is
STEP
1
2
3
O3(g) º O2(g) + O(g)
RAPID
O3(g) + NO(g)
NO2(g) + O2(g)
NO2(g) + O (g)
NO(g) + O2(g)
SLOW
FAST
a) Write the chemical equation for the overall reaction
O3(g) º O2(g) +
O3(g) + NO(g)
NO2(g) + O (g)
2O3(g)
O(g)
NO2(g) + O2(g)
NO(g) + O2(g)
3O2(g)
continued
35-8
b Give the rate law and order for the rate determining step.
Final Review
Rate law: Rate = k[O3][NO]
second order overall. 1st order w.r.t. [O3] and [NO]
c. Which chemical species is catalyzing the above mechanism ?
NO(g)
catalyst is regenerated
note in above NO2(g) is an intermediate: not asked for in that exam
35-9
Final Review
2. Consider the endothermic reaction
X(g)
Y(g)
Place one of the six letters A B C D E F corresponding to the terms
A) Reactant B) Product C) activation energy D reaction energy E transition state
F reaction intermediate in the appropriate box on the diagram. These terms refer to
the overall reaction as given. Use each letter once and once only. (Not all seven
boxes should be filled with a letter.
E
E
F
B
C
A
D
reaction coordinate
35-10
For the reaction A + B
C + D
the activation energy is 40 kJ mol-1 and )E (rxn) = -12 kJ mol-1
Final Review
The activation energy for the reverse process is:
a -40
b 28
c -28
d 52
e 12
solution
40
52
12
35-11
Final Review
The reaction A
B is simple second order in A with a rate constant of
2 X 10-2 L mol-1 s-1 at 25oC. The time required for 75% of the reaction of A is:
a 0.33
b 3.0
c 29
d 14
e not enough info to know
you need [A0] for second order
cannot find half-life without it
To determine the activation energy(Ea)one measures rate constants (k) at various
temperatures T (K) and plots which graph?
a k vs T b 1/k vs T c ln K vs T d ln k vs 1/T e none of these
The slope of the graph is equal to:
a -Ea b Ea c -Ea/R d Ea/R e none of these
35-12
For the reaction
Final Review
2Cr3+(aq) + 3IO4- + 10 OH-
2CrO42-(aq) + 3IO 3- (aq) + 5H2O(l)
under a given set of conditions the rate of appearance of CrO42is 3.0 x 10 -4 mol L-1 s-1. What is the rate of disappearance of IO 4-(aq)
a -4.5 x 10 -4 mol L-1 s-1
b 3.0 x 10 -4 mol L-1 s-1
c 2.0 x 10 -4 mol L-1 s-1
d 6.0 x 10 -4 mol L-1 s-1
e none of the above
1.5 times as much is used up
Carbon-14 decays with 1st order kinetics and a half-life of 5730 years. If 10 % of
the original amount of 14C is present in a sample the age of the sample is:
a 1.21 x 104 yr
b 1.21 x 10- 4 yr
c 8.26 x 103 yr
d 5.73 x 102 yr
e 1.90 x 104 yr
ln 10 = kt: k = 0.693/t1/2
ln 10 = 1.21 x 10-4 t
t = 19029
35-13
A reaction rate triples when the temperature is raised from 288 to 308K.
The activation energy is:
Final Review
a 0.240 kJ mol-1
b 40.5 kJ mol-1
ln 3 = Ea/R()T/T1T2)
c 17.6 kJ mol-1
d -40.5 kJ mol-1
e -17.6 kJ mol-1
N2O3(g)
Data
NO(g)
+ NO2(g)
[N2O3]
Initial Rate, mol L-1 s-1
1.5
2.0
3.0
4.0
0.112
0.265
0.896
2.12
try these two yourselves
the overall order is: a 1 b 2 c 3 d 4 e 0
solutions for this page at foot of 35-1
35-14